The technique of constructing high buildings has been developed remarkably in recent years. This development has been possible because of the use of building cranes that are able to operate at ever-increasing heights and hoisting capacities. Such development has very substantially reduced the manual labor required. Another desirable characteristic of modern building cranes is that they can be rapidly erected on a building site, and after the construction is completed, can be rapidly dismantled and transported to another building site.
A building crane for the construction of high buildings generally comprises a vertical derrick with a horizontally swinging jib at its upper end. The derrick consists of separate modular units that are joined together vertically to any desired height. Because of transportation limitations, the modular units must be limited as to their size and weight. In order to meet the limitations on reasonable transport weights and still achieve an increased hoisting capacity, crane manufacturers have tried to use high-strength and light-weight materials in the manufacture of cranes. One disadvantage has been that from a fatigue point of view such materials are generally not better then the material heretofore used. In some cases it has been possible to compensate for part of the fatigue by a more favorable design of the structural parts subjected to fatigue. However, one type of fatigue load which cannot be influenced by such measures and for which no solution was found so far, are stresses resulting from the oscillations of a crane.
A modern building crane can be constructed with an unstayed derrick to a substantial height. A relatively large part of the total mass of such a hoisting crane consists of the horizontally swinging jib supported at the upper end of the derrick. This swinging jib usually includes (a) a main jib having a carriage fitted with a load lifting device that is movable along the main jib and supported thereby and (b) a counter jib containing counterweights and hoisting machinery. The carriage which runs along the main jib usually is provided with a hook for raising or lowering a useful load. For such a crane, manufactured of high-quality steel, the rigidness of the whole structure is low. Loading and unloading operations give rise to oscillations imparting a translatory movement to the derrick by deflections and a rotary movement to the jib owing to angular variations at the top of the derrick.
It is generally known in mechanical engineering that oscillations can be damped out by coupling via a damping means to a weight mass. Known damping means may consist e.g. of a hydraulic or mechanical friction means.
This general knowledge has not been applied to the damping of crane derrick oscillations because as the derrick is extended as the building increases in height, the size of such a couterweight would also have to be changed.